The long-term goal of this proposal is to determine the function of the multiple endocrine neoplasia type-1 (MEN1) tumor suppressor gene and its importance to endocrine cell growth. Germline mutations of MEN1 occur in 77 percent of MEN1 patients and many sporadic endocrine tumors. Menin, the protein product of the MEN1 gene, is well conserved from mammals and rodents to more distantly related species such as zebra fish and drosophilae suggesting that it has an important evolutionary conserved function. Since menin has no significant homology to other known proteins, elucidating its function has been led by studies demonstrating protein interactions. Our yeast two-hybrid analysis with menin identified an interaction with a human homologue of the S. cerevisiae gene Dbf4 called ASK (activator of S-phase kinase). ASK functions as a regulatory subunit for the serine-threonine kinase, Cdc7. This evolutionarily conserved kinase complex is critical for origin firing and the initiation of S-phase during the G1-S transition. The primary substrate for ASK-Cdc7 is thought to be minichromosome maintenance protein, MCM2. ASK is cell-cycle regulated with maximal expression at Gl-S whereas Cdc7 is expressed relatively constant throughout the cell-cycle. This regulation of ASK and its association with Cdc7 is analogous to the relationship between cyclins and cdk's. Consequently, Cdc7 is referred to as a DDK (Dbf4 dependent kinase). Our preliminary data demonstrates that menin binds ASK and Cdc7, possibly as a ternary complex, and inhibits Cdc7 kinase activity as measured by ASK autophosphorylation. In this way, menin may be thought of as a putative DDK inhibitor (DDKI). Our hypothesis is that menin interacts with the ASK-Cdc7 complex, modulates Cdc7 kinase activity, and thereby regulates the initiation of DNA synthesis. This proposal will address this hypothesis with the following specific aims: 1. Determine the in vivo and in vitro interactions between menin, ASK, and Cdc7 and define factors important for their interaction. 2. Characterize the function of menin as a DDKI (Dbf4 dependent kinase inhibitor). 3. Identify physiological factors and their underlying molecular mechanisms important for modulating menin DDKI function.